#include<stdio.h>
#include<stdlib.h>

typedef struct Node {
  struct Node *next;
  char c;
}Node;

typedef struct List {
  Node *head;
  Node *current;
}List;

void pairwise_flip(List *l)
{
  Node *temp = NULL, *flip_temp = NULL;

  if(NULL == l || NULL == l->head) {
    printf("list or list-head is NULL.\n");
    return;
  }
  
  l->current = l->head;
  while (NULL != l->current) {
    temp = l->current->next;
    if (NULL != temp) {
      if(l->current == l->head) {
        l->head->next = temp->next;
        temp->next = l->head;
        l->head = temp;
      } else {
        flip_temp = l->head;
        while(flip_temp->next != l->current)
          flip_temp = flip_temp->next;
        
        l->current->next = temp->next;
        temp->next = l->current;
        flip_temp->next = temp;
      }
    }
    else
      break;
    l->current = l->current->next;
  }
}

void print_list(List *l)
{
  for(l->current = l->head; l->current; l->current = l->current->next)
  {
    printf("%c", l->current->c);
  }
  printf("\n");
}

void free_list(List *l)
{
  l->current = l->head->next;
  while(1) {
    free(l->head);
    l->head = l->current;
    if(NULL == l->current)
      break;
    l->current = l->current->next;
  }
  free(l);
}

int main(int argc, char *argv[])
{
  List *l = malloc(sizeof(List));
  int i = 0, list_size = 5;
 
  if (argc == 2) {
    sscanf(argv[1], "%d", &list_size);
  }

  if(list_size <= 0) return 0;
 
  l->head = malloc(sizeof(Node));
  l->head->c = 'a';
  l->head->next = NULL;
  l->current = l->head;

  for(i = 1; i < list_size; i++) {
    l->current->next = malloc(sizeof(Node));
    l->current = l->current->next;
    l->current->c = i+0x61; /* convert i to a char */
    l->current->next = NULL;
  }

  print_list(l);
  pairwise_flip(l);
  print_list(l);

  free_list(l);
  return 0;
}